Although the use of coal as a source of heat in pyroprocessing industries and electrical utilities has increased as a result of the increasing price and limited availability of oil and gas, very little effort has been expended to improve the combustion technology of this fuel. A typical system for burning coal, other than stoker firing, consists of a mill for grinding the coal to relatively small particle size, and a straight burner pipe into which the pulverized coal is introduced in a stream of air, the exit end of the burner pipe emitting the coal and air into a combustion chamber where firing occurs. In general, there are three basic methods of firing coal in this manner: direct fired, wherein the pulverized coal suspended in a stream of air (primary air) from the coal grinding mill passes through a burner pipe into the combustion chamber; semi-direct fired, wherein pulverized coal is removed from the mill airstream and is reinjected into the combustion chamber on a continuous basis; and indirect fired, wherein pulverized coal is collected from the mill airstream into storage, from which it is injected into the combustion chamber. In any of these coal firing methods, the coal is conventionally introduced into the combustion chamber through a straight pipe and suspended in a stream of "primary air".
In order to achieve the turbulence necessary for good combustion in the combustion chamber, it has been conventional to produce a high velocity air flow through the straight pipe, with consequent high pressure drop losses in the fan circuit supplying the stream of primary air from the coal mill. The requirement that the coal mill fan must create a high velocity flow through the burner pipe interferes with the efficiency of the coal mill fan, since the fan is being operated primarily for its effect on the burning of the coal in the combustion chamber, rather than for its primary function of providing proper air flow through the coal mill and coal classifier. In addition, the high velocity used to produce desirable turbulence in the combustion zone causes the coal particles to pass through the combustion zone before they can make effective contact with oxygen, thereby leaving some of the coal feed unburned.
There have been some efforts to achieve turbulence in the combustion zone by means other than a high velocity of the inlet coal particles, principally by swirlers placed in the burner pipe to create turbulence in the air stream. One such prior system involves the injection of preheated air directly into the primary burner pipe. This system created a problem with the operation of the coal mill in direct fired systems because the increase in static pressure in the burner pipe, resulting from the injected preheated air, could not be handled by the coal mill fan.
Another prior system included a burner that took a portion of air and coal from the coal transport pipe, removed the coal from the air by means of a cyclone and then returned the air to the burner tip at low temperature. This system was restricted in that the quantity and temperature of air it could utilize for controlling the flame was very limited. The system could handle only a portion of the already low air flow that the coal mill fan could deliver. It also lacked precise control of the limited air flow to the burner tip, and offered no source of high temperature to create combustion conditions.